The present invention is generally related to communication of digital data. More particularly, the present invention is related to communication of digital data among digital devices having different operating voltages.
Digital devices employ families of integrated circuits to operate on digital data and provide logical functions and operations. However, digital devices that must communicate may operate at different operating voltages. For example, some well-known logic families operate at a nominal voltage of 5.0 volts. Other logic families operate at nominal supply voltages of 3.3 volts or 1.8 volts. Some systems use combinations of more than one of these logic families.
When data is communicated among digital devices operating at different operating voltages, some transformation must be made to ensure reliable communication. The voltages generated by one device and corresponding to digital logic levels may be inappropriate for the input of another device. For example, a 1.8 volt supply device will have an output logic one level of approximately 1.8 volts. This may not be sufficient to be detected as a logic one at the input of a 5 volt supply device, which expects a logic one to have a value closer to 5.0 volts.
One accommodation for this problem is to provide pull-up resistors at the inputs of the 5 volt logic. If the voltage provided to the input is only driven to, for example, 1.8 or 3.3 volts, the pull-up resistor will pull the voltage to a value close to 5.0 volts to ensure accurate input level protection. However, pull-up resistors introduce a risk of latch-up, have poor noise immunity and reduced peak operating speed and increase overall current drain.
Another solution involves designing a cable that connects a 5 volt device and a lower voltage device such as a 3.3 volt device. The cable includes an active voltage conversion circuit for shifting input and output voltages. Such a cable tends to be an expensive alternative, however. The cable must be custom made for the application and must include the components of the active circuit.
Accordingly, there is a need for a method in apparatus for communicating digital data among devices having different operating voltages.